High-pressure sheets used as gasketing material have traditionally been prepared by mixing a base of fibers with a rubber binder and subjecting the resultant mixture to pressure and elevated temperature. Typically, such sheets have been made on a two-roll sheeter machine, such as a Troester machine, having a pair of rolls located one above the other. The lower roll has been typically provided as a larger, heated roll while the upper roll has been provided as a smaller, normally unheated roll. To the conventional sheet on a machine of this type, a quantity of starter compound is first built up on the hot roll. A quantity of body compound is then added in the nip between the two rotating rolls. A high pressure sheet material is formed on the hot roll to the desired thickness and can then be removed and placed on a table or other support.
The standard process for manufacturing high pressure sheets has involved mixing two separate component doughs, both of which have been essential components in making the high pressure sheet product. The main component, referred to as the body dough, has been mixed, for example, in a large drum mixer, although it has been known to be mixed in any of a variety of different mixer types. The components of this dough have typically included, for example, elastomers such as rubber materials, fibers, fillers such as clay, and small quantities of other materials, such as curing components, antioxidants and/or coloring agents which are known to those skilled in rubber compounding technology.
The beginning of the mixing process has typically involved introducing a pre-determined quantity of solvent into the mixer. Next, a pre-determined amount of elastomer, such as a rubber material, has likewise been introduced into the, mixer. The function of the solvent has been to dissolve the elastomer, thus allowing for uniform mixing with the other components in the body dough, and to aid in maintaining a proper viscosity of the body dough mixture, so that it builds properly on the hot roll during the sheeting process. Many solvents have been used in this process, including methyl ethyl ketone, heptane, and toluene, all of which are particularly hazardous.
Once the rubber material has been dissolved in the solvent, other components may be mixed in. The materials have been introduced into the mixer in different sequences, and depending on factors such as the materials used and the size of the mixture, a set time can be established for various steps of the operation. Once the mixture has achieved its desirable properties in terms, for example, of the state of the mixing, the quantity of solvent that is in the mix, etc., it is released from the mixer and transferred to the sheeting part of the operation.
Another important component of the high pressure sheet is a minority component which is referred to as the start coat. The start coat is a material which is formulated of materials very similar to the body dough formulation. A key characteristic of the start coat formulation is that it must adhere well to the hot roll of the sheet calendar in the manner described hereinafter. However, another key characteristic of the start coat formulation is that once the sheet has been built up to its desired thickness, the start coat must release easily from the calendar to provide a sheet that is very clean and smooth.
The calendaring operation begins once the start coat and body dough are transferred to the roll calendar. The two-roll calendar is a machine that is well known in rubber processing operations and consists of two rolls which are parallel to one another. Typically, one of the two rolls is heated to any of a variety of temperatures by internal heating components and a typical run temperature for the heated roll is from about 200° F. up to about 280° F. The other roll, known as the cold roll, can be cooled, for example, by water circulating within the roll. The material which is used to make the high pressure sheet material, i.e., the dough, is introduced into the nip between the two rotating calendar rolls.
A significant difference between the sheeting mill calendar and a typical rubber calendar has been that in a typical-rubber calendar, the rolls are held at a fixed gap between the two rolls and the material is squeezed in the gap between the two rolls, thus forming a rubber sheet of a pre-determined thickness. However, in the sheeting mill calendar, the gap between the two rolls is not fixed, but instead, the pressure on the two rolls is set such that the pressure applied at the nip is pre-determined. The sheet material, i.e., the dough, adheres to the hot roll thereby creating a back pressure against the two rolls. The pre-determined pressure at the nip between the two rolls is thus maintained as the dough adheres first to the hot roll and thereafter as additional dough adheres to the dough already deposited on the hot roll. Thus, while the pressure between the two rolls remains constant, the gap between the two rolls is allowed to increase as the material builds up to a desired thickness of the material.
At the commencement of the calendaring operation, an initial, relatively small quantity of the start coat formulation is introduced into the nip between the calendar rolls, which results in the adherence of a thin layer of the start coat, which can be as thin as 0.002 inch thick, to the heated roll. Thereafter; the body dough material is introduced into the nip between the calendar rolls and allowed to build up on top of the thin layer of start coat. The sheet that builds up on the hot roll can be thought of, for example, as a series of thin laminates adhering to one another. As the body dough material builds up on the heated roll, it forces the calendar rolls apart, and when the sheet which is building up reaches a desired thickness, the operation is stopped and the sheet is removed from the roll. The typical calendaring operation is a batch process in which one sheet at a time is formed with a width and length that is determined by the dimensions of the roll. In other words, the length and width of the sheet are determined by the length and circumference of the roll. However, the thickness of the sheet is allowed to build up under a pre-determined nip pressure until a desired sheet thickness is reached.
Typically, before the start coat is introduced, the calendar rolls literally touch one another, and as dough is introduced into the nip between the calendar rolls, the thickness of the sheet increases in very small increments on the order of 0.001 inch or less as the material builds up by adhering first to the heated roll and then to itself as thickness continues to build. It is noted that the sheeting operation is typically performed by a highly skilled operator who stops the operation when the desired sheet thickness is reached and uses, for example, a cutting device to cut the sheet along the length and circumference of the roll and removes the sheet from the roll and transfers it to a flat supporting surface.
A significant disadvantage of previously employed processes is found in the use of organic solvents, such as toluene, heptane, and methyl ethyl ketone, which have very low flash points and can be explosively flammable and which are also highly toxic. In addition, the explosive flammability of the material is greatly affected by any possible buildup of static charge in the dough and in the surroundings. It is readily apparent that as a static charge is built up, a spark can be created which can ignite an explosive fire. Thus, there is a present need for a process for making high-pressure sheet gasketing material that employs a less hazardous solvent than those previously used.
Another disadvantage of the currently employed process is the quality of the sheets which are produced by the process. A critical aspect of the process of building up the thickness of a sheet is ensuring that all of the material that goes into making the sheet builds up only on the heated roll and does not adhere to the cold roll. Any material which adheres to the cold roll will have the affect of producing a rough sheet. Thus, even a small amount of buildup on the cold roll will result, in a sheet that has a rough surface which is not only visually unattractive but also has a deleterious affect on the performance of the material, for example, in terms of sealability as a gasket material. Accordingly, there is also a present need for a process for making high-pressure sheet gasketing material with improved processability and sealing performance.